1. Field of the Invention
The present invention relates to power converters and more particularly to flyback DC-DC converters.
2. Description of the Related Art
Converters are frequently used to convert an unregulated voltage from a power source to a regulated voltage in electronic systems. Converters are especially prevalent in environments that include telecommunications or computer equipment, such as in power over Ethernet (PoE), potable electronic devices, etc. A converter can receive an input voltage and convert it to an output voltage or to a plurality of voltages at desired values according to users' requirements.
A conventional converter usually includes a transformer. The transformer typically includes a primary winding and a secondary winding. The primary winding is connected to a power source, preferably a DC voltage source to receive an input voltage that is unregulated. The transformer converts the input voltage at the primary winding to an output voltage at the secondary winding. The value of the output voltage is proportional to that of the input voltage, usually equaling the input voltage multiplied by winding ratio of the secondary winding to the primary winding. The energy from the power source can be stored in flux of the transformer core or be supplied to a load via the secondary winding. Hence, power transfer through the transformer is achieved.
According to one conventional approach, adjusting and controlling the output voltage value of the converter to satisfy different requirements may be desired. Accordingly, flyback converter topologies have been developed. A conventional flyback converter comprises a transformer with a primary winding and a secondary winding. Furthermore, an input circuit including a switching circuit is connected to the primary winding of the transformer while an output circuit is connected to the secondary winding of the transformer. A control block controls states of the switching circuit to control the cyclic switching on and off of the input circuit. A feedback loop couples the output circuit of the transformer to the input circuit of the transformer for purposes of supplying output voltage information to the control block. For purpose of isolating the input circuit and the output circuit of the transformer, the feedback loop of the conventional flyback converter usually employs an optical-coupler to supply the feedback information of the output voltage to the control block. Thus the control block can adjust the output voltage by controlling the power supply to the transformer according to the feedback information.
Controlling the power supply with precision is essential for effective power transfer in flyback converters. Because the input voltage is unstable and the winding ratio of the secondary winding to the primary winding is usually constant, the output voltage at the secondary winding will be unstable responsive to the input voltage. The stability of the output voltage ripple is not sufficiently ideal even though extra circuits such as filters may be employed to improve the performance of the converter. In addition, the load transient response in conventional converters is slow as the optical-coupler is coupled to the transformer to induce the feedback information of the output voltage to the control block of the transformer while isolating the input circuit and the output circuit of the transformer. As such, conventional converter topologies and methodologies perform unsatisfactorily.